Most present-day roof trusses form a triangular structure comprising a bottom chord and two inclined top chords which meet in an apex at a predetermined distance—rise—above the bottom chord. The bottom chord and the top chords are joined by web members forming triangular substructures of the roof truss. Each of the web members transmits one of a tension force and a compression force between one of the two top chords and the bottom chord depending on load acting on the roof truss.
Air in a well-ventilated attic space contains approximately a same amount of moisture as outside air. In winter the relative humidity of the outside air is relatively high, therefore, the top chords and web members absorb moisture until equilibrium is reached with the outside air. Consequently, the top chords and the web members lengthen.
The bottom chord, however, experiences a different phenomenon. In order to meet building code requirements of colder climate zones, builders cover the bottom chord with an approximately 300 mm thick layer of insulating material. Therefore, the average temperature surrounding the bottom chord is close to the indoor temperature. This causes the air adjacent to the bottom chord to have a much lower relative humidity than the air outside the layer of insulating material. As a result, the air adjacent to the bottom chord absorbs moisture from the wood causing the bottom chord to shorten.
As the bottom chord shortens and the top chords lengthen—which is not compensated by the lengthening of the web members—the apex of the roof truss is forced upward. Thus, web members connected to the top chords near the apex pull the bottom chord upward resulting in a roof truss uplift causing cracks of up to approximately 20 mm width between ceilings and partitioning walls. It is worth noting, that in case the chords and the web members are made of compression wood or juvenile wood, this effect is significantly increased.
Some builders mask the effects of the roof truss uplift by securing the ceiling drywall to the top of the partitioning walls and not to the roof trusses for a distance of approximately 45 cm from the partitioning walls. The drywall flexes and stays fastened to the partitioning walls while the trusses lift above it. Unfortunately, this method leaves a considerable portion of the ceiling drywall without support. Furthermore, mounting of fixtures such as hanging light fixtures to this portion of the ceiling is difficult if not impossible.
Others fasten decorative moldings to the ceilings along edges where the partitioning walls and the ceilings meet. As the ceilings move up, the moldings move therewith, thus hiding the gap. Unfortunately, since colors change when exposed to sunlight this will expose an undecorative stripe below the molding in winter. Furthermore, it is suggested to always redecorate in winter when the ceiling is at its highest point. Otherwise a stripe will be exposed below the molding during the following winter.
It would be highly desirable to overcome these drawbacks and to substantially reduce roof truss uplift.